In IEEE802.11 standards, access control is performed such that a plurality of terminals can share the same frequency. As an example of the access control, carrier sense multiple access with collision avoidance (CSMA/CA) is adopted. CSMA/CA is a scheme in which, after each wireless terminal station which is about to transmit a frame performs carrier sense (check the state of a wireless channel) in advance, the wireless terminal station starts transmission by using control called random backoff control by which a collision between transmit data is avoided. The random backoff control is control by which a wireless terminal station that holds transmit data performs carrier sense and, after a channel used for data transmission enters an idle state for a distributed coordination function inter frame space (DIFS) period, the wireless terminal station generates random numbers in a specified contention window (CW) range and determines a random backoff time based on the random numbers.
Moreover, if transmit timing of one wireless terminal station collides with that of another wireless terminal station, the CW range is further broadened and retransmission is performed, whereby a collision of transmit timing is avoided. In the IEEE802.11a standards, the CW size can be increased in six stages from 15 to 1023. Furthermore, when the CW size reaches the maximum value, the CW range is not broadened and is made to remain at the maximum value until the number of retransmissions reaches a previously determined maximum number of retransmissions. If retransmission is unsuccessfully performed even when the number of retransmissions exceeds the maximum number of retransmissions, the transmit frame is discarded.
A model expected by the existing IEEE802.11 standards is a model in which, in uplink communication, one node repeatedly performs transmission to an access point (AP) in sequence. Therefore, in the IEEE802.11 standards, in order to prevent a plurality of terminals from performing transmission to the AP at the same time and causing a collision, a technology called the distributed coordination function (DCF) using CSMA/CA described above is adopted. In the DCF technology, a wireless terminal station that holds transmit data sends request to send (RTS) to the AP and the AP sends clear to send (CTS) to the wireless terminal station in response to that, whereby a collision of transmit data which may occur due to a hidden terminal is avoided. This stream of processing is called RTS/CTS exchange, and RTS/CTS exchange is performed based on dot11RTSThreshold. The RTS/CTS exchange is not performed when a frame which is shorter than dot11RTSThreshold is transmitted.
As described above, in the IEEE802.11 standards, the DCF technology by which a collision of the transmit timing of wireless terminals is minimized is adopted.
Moreover, recently, in uplink communication, the multi user-multi input multi output (MU-MIMO) technology by which a plurality of terminals transmit data at the same time by using the same channel has been studied. In the MU-MIMO technology, what is important is that a plurality of terminals transmit data with the same data transmit timing and the same frequency band is used.
In a technology described in PTL 1 described below, MU-MIMO is implemented by a shape expanding the DCF (by performing additional RTS/CTS exchange).
Moreover, in a technology described in PTL 2 described below, concurrent transmission is implemented by control of a backoff value.